The need to align an object repetitively arises in a number of manufacturing processes. For example, in the fabrication of integrated circuits, each semiconductor device results from a series of chemical and depositional processes. The success of the fabrication process depends on the wafer being in a known position when each of these process steps is performed. If the wafer is misaligned during one of these process steps, the resultant integrated circuits may be defective.
The precise positioning of the wafer in the processing equipment is typically accomplished by providing a number of fiducial marks on the surface of the wafer. The wafer is positioned utilizing a "coarse-fine" positioning scheme. The coarse adjustments position the wafer such that one or more of the desired fiducial marks are within the field of view of a scanning device. The position of the desired fiducial mark is located by scanning the surface of the wafer in the vicinity of the fiducial mark being sought. The signal from the scanning device is then processed to determine the location of the fiducial mark Once such a determination is made, fine adjustments in the position of the wafer are made.
The accuracy of the fine adjustment depends on the precision with which the location of the fiducial mark can be determined. When the waveform from the scanner is not corrupted by noise, the location of the fiducial mark can be determined by relatively simple computer algorithms. For example, one prior art algorithm detects the peak of the signal from the scanning device and then locates the points at which the signal crosses one or more threshold values. The threshold values are set in relation to the peak height. In the simplest form of thresholding algorithm, a single threshold value is utilized, typically, one half the peak height. The fiducial location is then assigned to a location computed from the average of the locations at which the signal crosses the threshold value.
This type of simple prior art scheme for locating the fiducial marks fails when the signal is corrupted by noise. Noise can result from electronic noise or from surface imperfections on the wafer in the vicinity of the fiducial mark. Such imperfections can result from damage during previous processing steps or from particulate matter on the surface of the wafer. In any event, if the signals arising from this noise are above the threshold value in locations off of the fiducial mark, these signals will lead to errors in the determination of the fiducial location. In addition, noise which reduces the signal value at locations on the fiducial mark also may lead to errors in fiducial location. Such noise can reduce the signal intensity in the fiducial region to a value below the threshold value.
A second problem with this type of fiducial mark location algorithm results from the need to set a threshold value. If the fiducial marks vary in "brightness" across the wafer, the correct threshold value in one portion of the wafer will differ from that in another portion of the wafer. In addition, the brightness of any given fiducial mark typically changes in the course of wafer processing.
A number of prior art solutions to the above mentioned problems with this type of simple fiducial mark location scheme have been proposed. For example, Murakami, et al. ["Laser Step Alignment for a Wafer Stepper", SPIE 538 Optical Microlithography IV (1985)]teach a scheme in which the fiducial marks consist of small diffraction gratings. These gratings are scanned utilizing a laser. Since the diffraction grating properties of the fiducial marks are utilized in recognizing the marks, problems arising from noise or surface imperfections which would otherwise give rise to "false" fiducial marks are reduced.
However, this type of scheme has a number of significant drawbacks. First, if the fiducial mark has a rough surface, is damaged during processing, or is corrupted by particulate matter, the diffraction grating effect may be substantially reduced. The accuracy with which the fiducial mark may be located is then significantly reduced. Second, the method requires a different type of detection apparatus from that included in many alignment stages. Hence, a significant cost must be incurred to retrofit existing equipment if this method is to be utilized.
A second prior art scheme for improving fiducial mark detection is taught by G. Owen ["Shot Noise Errors in Registration for Electron Lithography," J. Phys. D : Appl. Phys., 19, pp. 2209-2223, (1986)]. In this scheme, a fiducial mark consisting of a trench etched in the substrate surface or of some other feature constructed by depositing a heavy metal on the surface of the substrate is utilized. The fiducial mark is scanned with an electron beam and the back-scattered electrons are detected as a function of the electron beam position to provide a registration signal.
The location of the fiducial mark in the registration signal is detected by finding the maximum of a detection function constructed by convolving the registration signal with a "template" representing an ideal, noise-free, registration signal. This convolution is calculated for each possible position of the fiducial mark relative to the start of the electron beam scan. The fiducial mark is assigned a location determined from an analysis of the convolution function the vicinity of the maximum of the convolution function. The simplest form of analysis assigns the fiducial mark location to the maximum of the convolution. However, it will be apparent to those skilled in the art that a thresholding algorithm similar to that used in analyzing the measured data in prior art devices may be used to analyze the convolution function.
Although this scheme provides improved noise immunity relative to the simple threshold scheme described above, without requiring specialized detection equipment, its accuracy is still dependent on having a relatively "clean" fiducial mark. If the fiducial mark is damaged in a manner which causes its detection signal to differ significantly from that expected from an ideal fiducial mark, significant location error can occur.
Broadly, it is the object of the present invention to provide an improved method for locating a fiducial mark in a signal.
It is a further object of the present invention to provide a method which is less sensitive to noise than prior art methods.
It is yet another object of the present invention to provide a method that is less sensitive to distortions in the signal which cause the signal to differ from an ideal signal.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.